feat(wallpaper): complete IPC and lock synchronization

[bhdai]

Summary

• complete the six-function wallpaper IPC contract with live library cycling, shared decode-before-commit per-monitor overrides, immediate clears, and required warnings
• keep desktop resolution live through Wallpaper.forMonitor(...) and bind each lock surface to its own raw screen name
• extend the offscreen service fixture across cycle, override, clear, warning, persistence, no-op, and live-reload behavior

Companion non-repeating keybinds: bhdai/dotfiles#4.

Closes #104. Parent spec: #96.

Verification

• node --test
• tests/wallpaper-service.sh
• git diff --check

Manual nested verification required

No visual or compositor-runtime claim is made from this environment. With the session unlocked, run ./scripts/dev-nested.sh and confirm:

• connected and disconnected raw output names resolve and retain per-monitor choices
• each desktop and lock surface follows its own resolved wallpaper
• the HDMI-A-1 mirror behavior is acceptable
• a wallpaper change while locked hard-cuts without exposing the desktop or affecting authentication
• empty, one-item, unknown-current, and wrapping next/prev behavior matches the returned paths